Metal Detectors: How They Work

You see them on the beach or floating daintily over the grass in neighborhood parks. They have a mysterious ability to sense metal several inches below the ground or water. Yet these amazing devices are based on a very simple principle that a coil of electricity induces a magnetic field.

The disk on the end of a metal detector is actually a coil of metal inside a plastic case. A pulse of electricity is sent through it from a power source. The electric pulse creates a magnetic field which is projected all around it. Even as an electric current can create a magnetic field, a magnetic field, when it comes in contact with metal can create a current.

Just so, when the magnetic field (created by the coil) passes through a bunch of coins (or any other metalic item) it creates a current in them - these currents are known as Eddy Currents. They, in turn, create their own magnetic field, affecting the metal coil in the metal detector (actually creating an opposite current to the pulse). Sensors detect this opposite current and send a warning signal to the operator of the metal detector either through light or sound.